1. Field of the Invention
The invention relates to a method for analyzing and certifying the composition and/or properties of manufactured petroleum products.
2. Description of Related Art
Many petroleum refinery products such as gasoline and diesel fuel are typically manufactured. For one example, several petroleum refinery streams that differ in composition and properties may be blended to form a petroleum fuel product. For another example, the petroleum product may be subjected to treatments such as sulfur removal or additive injection. In most instances the final product must meet regulatory and/or contractual requirements. Typically, the overall objective of the manufacturing process is to produce a regulatory or contractually compliant product, while minimizing the overall cost of production.
In many manufacturing processes, the composition and properties of the components, such as the blend component streams, are used as input to a set of blending equations (a.k.a., a blending model) to estimate a blend recipe that ensures the blended or manufactured product meets targeted composition or property values. The actual composition and properties of the blended product however do not always agree with that calculated from the blend recipe. Various reasons for this discrepancy exist including but not limited to: errors in the composition/property estimates for the blend components; errors in the blending model; errors in the blending flow control; random and systematic errors in the measurement systems used to control and certify the final blend properties.
To assist in the manufacture or blending of petroleum refinery products, a Flow Proportioned Average Property Value (FPAPV) for the aggregated manufactured product is often calculated. The FPAPV values are based on the integration of process analyzer values and the measured flow of the manufactured product that make up the aggregated volume of the manufactured product (using for example ASTM D6624). The FPAPV is typically calculated for each compositional and property parameter.
FPAPV values are, in conjunction with the instantaneous (on-line process) analyzer values, continuously compared to targets, and the relative component ratio or other key manufacturing process parameters can be adjusted accordingly. These calculated values, however, are not currently acceptable for certifying that a product meets EPA regulatory specifications, and, may not be acceptable to some customers as the supplier certified value to deem the product meeting specification requirements.
Instead, when product is blended to tank or otherwise manufactured, generally accepted practice is to take samples of the product from the tank (typically top, middle and bottom) and analyze the samples in a laboratory (i.e., off-line instrumentation). Usually the samples are collected upon completion of the manufacture. The analysis is typically performed using EPA designated methods and/or contractually specified methods to determine if product specification and/or EPA requirements are met. For instances where the manufactured product is blended directly to pipeline, barge, ship or tank car, ASTM D 4177 (used in conjunction with D 5482 for volatility measurement) is used to collect a composite sample that is representative of the blend. For regulatory and contractual compliance, this composite sample is analyzed in a laboratory using EPA designated or contractually specified methods with off-line instrumentation for final certification purposes upon completion of manufacture. The measurement methodology used in the off-line instrumentation is usually different from that used with the on-line process analyzer during the manufacture of the petroleum refinery product.
In many manufacturing processes of petroleum refinery products, the producer establishes a manufacture or blending target for each composition or property variable for which there is a regulatory or contractual specification. In order to control the risk (probability) that the final certification measurement will show the product to be off-specification, (thereby incurring business penalties associated with re-work, product downgrade, or missed shipment), it is generally necessary to adjust manufacturing or blend targets farther from the regulatory or specification values, on the compliant side. These adjusted targets account for the overall variability of the planning, manufacturing, sampling, and testing processes. Meeting these adjusted targets generally requires use of higher proportions of more expensive blend or manufacturing components, thereby increasing the manufactured cost of the petroleum product.
A significant portion of the overall variability described in the process can be attributed to the variation of the disagreement (difference) between on-line analyzer generated values (instantaneous and FPAPV) that are used to control the manufacturing process, versus the final laboratory (off-line) certification results. The variability of this difference is quantified by the cross-method reproducibility (reference ASTM D 6708) between the on-line process analyzers and off-line laboratory methods. Blend or manufacturing targets are adjusted farther away from the regulatory and contractual specification values to account for the cross-method reproducibility between the on-line process analyzers and off-line laboratory methods. The difference between the manufacture target and the specification limit is often referred to as the manufacturing offset. In general, the cost of manufacture of the product is directly proportional to the magnitude of the manufacturing offset. Hence, there is a strong economic incentive for the manufacturer to minimize such offsets.
To reduce the manufacturing offset (and hence the associated cost) of manufacturing petroleum products that are compliant with contractual and/or regulatory specifications, there is a need for a method that minimizes the variability in the blending and/or manufacturing of petroleum refinery products. It would be desirable to have a method of certifying the composition or properties of a manufactured petroleum product that reduces or eliminates the cross-method reproducibility contribution towards the manufacturing offset.